MLMs are commonly employed in optical devices to obtain high reflectivity at a free-space operating wavelength .lambda.. In VCSELs, for example MLMs may comprise dielectric (i.e., non-semiconductor) materials as shown in FIG. 1, or one MLM may comprise dielectric materials and the other semiconductor materials as shown in FIG. 2. In either case, an MLM comprises a plurality of pairs of quarter-wavelength thick layers, each pair comprising a relatively high refractive index (n.sub.H) layer adjacent a relatively lower refractive index (n.sub.L ) layer. Depending on the refractive index difference between these layers, just a few pairs of layers can yield reflectivities greater than 90% over a wavelength range .DELTA..lambda. of several hundred nanometers. The reflectivity, R, and the spectral width, .DELTA..lambda./.lambda., of the MLM are given by equations (1) and (2), respectively: EQU R=tan h.sup.2 [0.5 ln (n.sub.B /n.sub.M)+N ln (n.sub.H /n.sub.L)] (1) EQU .DELTA..lambda./.lambda.=(4/.pi.) sin.sup.-1 [(n.sub.H -n.sub.L)/(n.sub.H +n.sub.L)] (2)
where N is the number of pairs of layers in the MLM. In addition, the MLM is assumed to be disposed between a body having a refractive index n.sub.B and a medium having a refractive index n.sub.M. Illustratively, the medium is air, an index-matching epoxy or an optical fiber. into which the output beam of the VCSEL is coupled. Equations (1) and (2) also assume that inequality (3) is satisfied: EQU n.sub.B.gtoreq.n.sub.H &gt;n.sub.L.gtoreq.n.sub.M (3)
and that the order of the pairs is such that the lower index layer of the first pair contacts the body and the higher index layer of the last pair contacts the medium. The set of equations for other cases can readily be derived from the above by those skilled in the art.
Several combinations of dielectric mirrors are well known for use in MLMs, with the choice for a particular application often being based on one or more of the following criteria: (1) ease and uniformity of deposition, (2) environmental safety, (3) reflectivity and spectral width, and (4) reproducibility. One combination of materials known in the prior art utilizes ZnS as the higher refractive index layer (n.sub.H =2.30) and a fluoride (e.g. CaF.sub.2 or MgF.sub.2) as the lower refractive index material (n.sub.L =1.38). Theoretically an MLM made of only six to eight pairs of these layers could produce extremely high reflectivities of 99.5% to 99.9% at .lambda.=0.7-1.5 .mu.m, making such MLMs very attractive for use with many VCSELs; e.g., GaAs/AlGaAs VCSELs as well as InGaAs/GaAs VCSELs. In practice, however, we have found that several problems are encountered with either ZnS/MgF.sub.2 or ZnS/CaF.sub.2 MLMs: high light-scattering losses or serious patterning difficulties.
Light scattering is attributable to non-smooth layer surfaces or to small amounts of crystallization in the otherwise amorphous dielectric materials. Just a fraction of a percent loss of reflectivity due to such scattering may be intolerable in some VCSEL designs, and cannot be compensated by including additional pairs of layers in the MLM. On the other hand, patterning difficulties arise because the fluorides and ZnS have different chemical properties, making it difficult to etch the MLMs controllably. Of course, MLMs which cannot be controllably patterned are impractical for optical devices such as VCSELs because they would inhibit or prevent the formation of electrical contacts to the underlying material of the laser (e.g., the semiconductor material of a VCSEL).